Electrostatic pinning of dielectric film

ABSTRACT

A PROCESS OF ELECTROSTATICALLY PINNING A MOVING DIELETRIC WEB TO A ROLL WHICH COMPRISES ESTABLISHING A POTENTIAL DIFFERENCE BETWEEN A PINNING WIRE AND A SECOND ELECTRODE INDEPENDENT OF THE ROLL, AND PREFERABLY SURROUNDING AT LEAST THE FIRST ELECTRODE WITH A GAS IN WHICH A SPECIFIC MINIMUM CURRENT CAN BE GENERATED.

y 2, 1972 w. E. HAWKINS ELECTROSTATIC PINNING OF DIELECTRIC FILM Flled March 23 1970 3 Sheets-Sheet 1 F I6. 2

' WILLIAM E. HAWKINS ATTORNEY y 2, 1972 w. E. HAWKINS 3,560,549

ELECTROSTATIC PINNING OF DIELECTRIC FILM Filed March 23, 1970 3 Sheets-Sheet 2 INVENTOR WILLIAM E. HAWKINS @M&% l

ELECTROSTATIC PINNING' OF DIELECTRIC FILM Filed March 123, 1970 3 Sheets-Sheet 5 INVENTOR WILLIAM E. mums AT'T NEY United States Patent Ofice Patented May 2, 1972 3,660,549 ELECTROSTATIC PINNING OF DIELECTRIC FILM William E. Hawkins, Circleville, Ohio, assignor to E. I. du Pont de N emours and Company, Wilmington, Del. Continuation-impart of application Ser. No. 823,810, May 12, 1969. This application Mar. 23, 1970, Ser.

Int. Cl. B29d 7/02 U.S. Cl. 264-22 12 Claims ABSTRACT OF THE DISCLOSURE A process for electrostatically pinning a moving dielectric web to a roll which comprises establishing a potential difference between a pinning wire and a second electrode independent of the roll, and preferably surrounding at least the first electrode with a gas in which a specific minimum current can be generated.

CROSS-REFERENCE TO RELATED APPLICATION This is a continuation-in-part of copending application Ser. No. 823,810 filed May 12, 1969, now abandoned.

BACKGROUND OF THE INVENTION In the preparation and treatment of dielectric films, it is often necessary to apply a force to the film to bring or hold it in contact with rolls or belts. For example, films are regularly subjected to coating and stretching treatments where firm contact with rollers is essential to the operation, but where the application of conventional mechanical forces would be undesirable. One of the most widely used methods of pinning film involves imparting an electrostatic charge to the film through an electric field between wire or point electrods and the grounded surface on which the film is carried. However, for certain high-speed operations, even electrostatic pinning is less than completely satisfactory.

Certain unique requirements have been encountered, for example in the casting of molten, crystallizable thermoplastic web, where it is necessary to quickly cool the molten web to a temperature below the glass transition temperature to minimize crystallization. The extruded web is generally cooled by casting the molten thermoplastic material onto a moving chilled surface, using electrostatic pinning to bring the web into intimate contact with the chilled surface. Previous attempts to increase the speed of this procedure for more efiicient and economical operation have resulted in poor gauge uniformity and regularly recurring haze paterns known in the art as venetian blind haze.

The higher speeds are believed to result in the entrapment of air between the drum and the web, which hinders quenching by diminishing heat transfer between the drum and the web. Attempts have previously been made to increase the electrostatic force generated by the wire or probes by increasing the voltage. These attempts, however, have for the most part been inifective, since increased voltage generally causes a catastrophic electrical breakdown between the electrode and the web long before any substantial increase in the pinning force is effected. The sparking between the electrode and the surface of the web or other parts of the apparatus interrupts the electric field which contributes to the pinning force. In addition, the sparking can cause pinholes in a freshly cast soft web, which holes are greatly enlarged by an orientation of the film.

Consequently, the pinning force available through electrostatic pinning means has heretofore been inadequate for some high-speed applications.

SUMMARY OF THE INVENTION The instant invention provides a method of electrostatic pinning which results in a substantially higher and more uniform pinning force than has heretofore been available.

Sspecifically, the instant invention provides a process for pinning a dielectric film to an electrically grounded moving surface which comprises (1) establishing an electrical potential difference between an electrode pair comprising a first electrode and a second, grounded electrode by connecting the first electrode to a high voltage current source, the first electrode being in spaced relationship to the moving surface, the distance between the first and second electrodes being less than the distance between the first electrode and the moving surface and the second electrode being substantially fully insulated from the first electrode by dielectric; and (2) passing the film in proximity to but out of contact with the electrode pair and onto the surface, causing the film to adhere to the surface. Preferably the process further comprises substantially surrounding at least the first electrode with an atmosphere consisting essentially of a gas in which a wire current before breakdown of at least about microamperes/inch of wire can be generated, as measured by the current generation test.

BRIEF DESCRIPTION OF THE DRAWINGS FIGS. 1, 2, 3 and 4 are schematic representations of cross-sectional views of apparatus arrangements which can be used in the instant process, shown in conjunction with extrusion and quenching apparatus.

FIG. 5 is a view in perspective of an apparatus of FIG. 3.

FIGS. 6, 7 and 8 are graphical representations of the gauge variations of films prepared according to the instant process compared with films pinned according to methods previously used.

FIGS. 9 and 10 are cross-sectional schematic representations of additional apparatus arrangements which can be used in the inveniton.

DESCRIPTION OF THE PREFERRED- EMBODIMENTS The first electrode used in the process of the instant invention can be made from any metallic conductor having adequate strength and dimensional stability to withsatnd the stresses encountered in the operation of the instant process. Such materials can include, for example, tempered steel, tungsten, Inconel nickel-iron alloy, Monel nickel alloy, copper, brass, and stainless steel. The particular configuration of the first electrode can be similar to that of electrodes generally used in the art of electrostatic pinning, such as those described in Owens et al., United States Pat. 3,223,757, hereby incorporated by reference. For full sheet electronstatic pinning, a thin wire, substantially circular in cross-section, is especially preferred due to the exceptionally high rate of ion formation believed to result from the minimum surface area. However, in those embodiments of the invention using a knife edge or a plurality of pointed electrodes as described in the Owens et al. specification, the functional edge of the electrode should be directed toward the second electrode required in the instant process instead of the quenching surface as required in Owens et al.

In the instant process, the primary electric field for the generation of pinning ions is established between a first electrode, described above, and a second, grounded electrode. Accordingly, an important feature of the process of the instant invention is that the distance between the first and second electrodes be less than the distance between the first electrode and the grounded moving surface. Outside of this basic requirement, the positioning of the second electrode can vary widely. However, since the point of ion formation is believed to be at or near the first electrode, the straight linear paths between the first electrode and the grounded moving surface should be substantially unimpeded, particularly with respect to those portions of the moving surface prior to the touchdown point or area of maximum pinning force. Thus, while a rod-like or relatively small second electrode can be beneficially placed between the first electrode and the grounded moving surface for the acceleration of ions toward that surface, it is generally preferred that the second electrode be placed farther from the thermoplastic web than the first electrode so as to minimize interference with the deposition of ions onto the web. It is also preferred that the distance between the first and second electrodes be less than about one inch, for optimum ion formation.

The second electrode can be prepared from the same materials suggested above for use in the first electrode. In addition, the second electrode is substantially fully insulated by dielectric material. In accordance with the instant invention, the second electrode is considered substantially fully insulated when substantially all of the straight linear paths between the first and second electrodes are intersected by dielectric. The dielectric insulation of the second electrode prevents the discharge of gaseous ions on the second electrode and enables the application of exceptionally high voltages to the first electrode without a breakdown in the gap between the electrode pair.

Dielectric materials which can be used to insulate the second electrode include natural and synthetic rubbers as well as resinous materials such as polyimides, fluorocarbon resins, urea formaldehyde resins, phenol formaldehyde resins, nylons, and cast epoxy resins. It has been found that T efion fluorocarbon resins are particularly well suited for this application due to their excellent electrical insulating properties and stability at elevated temperatures.

The particular physical configuration of the second electrode is not critical to the instant process, as long as the second electrode does not surround the first electrode to the extent of intersecting a substantial percentage of the straight linear paths between the first electrode and the moving surface. Accordingly, the second electrode, in a basic form, can be a metal rod, as illustrated in FIG. 1. In that figure, a thermoplastic Web is extruded from hopper 11 onto a moving quench drum 12. A first electrode 13 and a second electrode 14 are positioned approximately above the touchdown point 15 of the web onto the drum. The first electrode is connected to high voltage source 16 and the second electrode is insulated by sheath 17.

The second electrode, in a preferred embodiment of the invention, is arcuate in cross section and is positioned farther from the web than the first electrode, with the first electrode positioned within the arc defined by the second electrode. This particular embodiment facilitates the direction of the ions formed by the electric field toward the thermoplastic web. One such embodiment is illustrated in FIG. 2, wherein arcuate second electrode 18 is positioned adjacent first electrode 13 and is insulated therefrom by a dielectric insulating material 19. The semicircular configuration of the second electrode permits the use of an insulating barrier on only one side of the second electrode, while intersecting all of the straight linear paths between the electrode pair.

In still another embodiment of the invention, illustrated in FIG. 4, a terminal end 20 of the arcuate electrode is uninsulated, in that there is a straight linear path between the first electrode and a portion of the second electrode not shielded by dielectric material. The surface area of any such uninsulated portions of the second electrode should be minimal, since the gaseous ions formed at the first electrode, if drawn into contact with the uninsulated portion of the second electrode, are discharged, instead of retaining their positive or negative charge and repelling like ions toward the web to effect the desired pinning. Another disadvantage of uninsulated sections on the second e1ectrode is the increased tendency for interruption of the pinning force through electric breakdown between the first electrode and the uninsulated portions of the second electrode. This effect can be minimized by rounded and polished metal surfaces on any uninsulated portions of the second electrode to decrease the available sites for spark initiation.

The positioning of any uninsulated portions of the second electrode is critical to the extent that the pinning force will be nullified or substantially diminished if the uninsulated portion is positioned, with respect to the first electrode, either diametrically opposite the site at which the deposition of ions is desired or directly between the first electrode and the site of deposition.

In addition to the higher pinning force obtainable through establishing a potential between the two electrides in accordance with the instant invention, another advantage can be realized through a specific position of the second electrode. It has been found that the charging of a freshly extruded web along the span extending from the hopper lip to the touchdown point increases the attraction of the web to the quenching surface so as to result in increased film. This variation in the touchdown point can result in the intermittent entrapment of small air bubbles, an effect often encountered at high quenching speeds. Accordingly, the second electrode is preferably positioned so as to prevent charging of the web substantially before the touchdown point onto the drum. For example, the second electrode can be positions so as to intersect at least about 50% of the straight linear paths between the first electrode and that portion of the web extending from the die orifice to the point of touchdown of the film onto the quenching surface. Such positioning is illustrated in FIGS. 9 and 10 wherein electrodes 14 and 18, together with insulating layers 17 and 19 are positioned so as to intersect a substantial percentage of the straight linear paths between wire electrode 13 and the freshly extruded web '10. It may be noted that this positioning does not interfere with the straight linear paths between the first electrode 13 and the quenching surface 12, the extreme linear paths being defined by tangents to the quenching surface from the wire.

A gap should be maintained between the second electrode and the freshly extruded web sufiicient to prevent the web contacting the second electrode due to the aerodynamic etfects of the moving web. A set gap of at least about A;" is generally sufficient to prevent contact.

Both the second electrode and the moving surface in the instant process are generally grounded to provide the requisite potential difference between the first and second electrode and the attraction of the formed ions by the drum. The grounding can be direct or through nominal resistance, such as the framework of the mechanism involved.

The high voltage source applied to the first electrode can be any current source having a voltage of about from 2 to 30 kilovoltes and an amperage of about from 1 to 3000 microamperes, for electrodes generally used for pinning operations. In general, a unidirectional current source, and especially a positive unidirectional current source, are preferred.

In a preferred embodiment of the instant invention, a gas stream is directed toward the thermoplastic web so as to substantially surround at least the first electrode. The gas is believed to contribute to the pinning force by increasing the current generated on the wire without sparking. Accordingly, gases which can be used in this embodiment should be selected from those in which a wire current before breakdown of at least about microamperes/inch of wire can be generated, as measured by the current generation test. This test, together with specific gases satisfying this requirement, are described in copending, coassigned United States Patent application Ser. No. 21,696, filed simultaneously herewith, and hereby incorporated by reference.

It is preferred that the difference between the voltage at threshold current and the voltage at spark breakdown of the gas be at least 2.0 kilovolts, also as measured by the current generation test. This minimizes the need for close control of the voltage applied to the pinning apparatus to prevent sparking.

Representative gases in which a wire current before breakdown of at least 100 microamperes per inch of wire can be generated include nitrogen, helium, air having a moisture content of less than about hereinafter referred to as dry air, oxygen, dichlorotetrafluoroethane, (Commercially available from E. I. du Pont de Nemours and Company as Freon 114.) dichlorodifluoromethane, (Commercially available from E. I. du Pout de Nemours and Company as Freon 12.) carbon tetrachloride vapor in dry air or nitrogen, tetrachloroethane vapor in nitrogen and acetone vapor in nitrogen. The carbon tetrachloride, tetrachloroethane and acetone vapors can be obtained by bubbling the vehicle gas, e.g., dry air or nitrogen, through a liquid bath at room temeprature, until the vehicle gas is substantially saturated with the vapor.

In those embodiments of the invention where the second electrode is of a simple rod-like configuration, the gas stream is beneficially directed so as to intersect both electrodes. In those embodiments where the second electrode is of a semicircular or arcuate configuration, the gaseous stream is best directed so as to surround the first electrode as well as the insulated surface of the second electrode facing the first electrode. An apparatus which can be used for this lattermost embodiment is illustrated in FIG. 3, wherein a gaseous stream 30 supplied from gas source 31 is passed through second electrode 32 and dielectric insulator 33 to escape along the surface of the insulating layer and over first electrode 13 onto the thermoplastic web.

The gas should be supplied at a minimal pressure necessary to maintain an atmosphere around the electrodes, since extremely high gas pressure can cause distortion of the web in some applications.

The process of the instant invention is applicable to the pinning of any dielectric film. Preferred films include those of organic thermoplastic polymer including, for example, polyesters such as polyethylene terephthalate, polyolefins such as polyethylene and polypropylene, vinyl acetate polymers and copolymers, vinylidene chloride polymers and copolymers, polyamides, cellulosic esters and ethers, styrene polymers and copolymers, rubber hydrochlorides and polycarbonates. The process is particularly applicable to the quenching of crystalline polymers and especially polyethylene terephthalate, since the drawing and resulting optical properties of these films, when produced at high speeds, are greatly enhanced.

The instant process is additionally applicable to the handling of other dielectric films, for example, in coating or printing operations for papers, cellophane, and thermosetting resins such as Kapton polyimide resin. I

The process of the instant invention provides a higher and substantially more uniform pinning force than has heretofore been available through the use of known methods of electrostatic pinning. The reasons for the improved pinning are not fully understood, but are believed to be primarily a function of the independence of the pinning apparatus from the grounded roll to which the thermoplastic fihn is pinned and the insulation of the second electrode. The instant process is found to provide increased pinning force even when positioned substantially farther away from the thermoplastic web than has been possible using pinning processes wherein the primary electrical field is established between the electrode and a grounded roll. Further, the process is relatively independent of external mechanical vibration, resulting in a more uniform generation and deposition of gaseous ions onto the thermoplastic film.

The benefits of the instant process are further illustrated in the following specific examples.

EXAMPLE 1 A control standard is established by extruding polyethylene terephthalate and pinning it to a quench drum substantially as described in Example 1 of Owens et al., U.S. Pat. 3,223,757, and thereafter biaxially stretching it to a thickness of about 2.0 mils. The thickness variations of the oriented film along the transverse and machine direction axes are determined and plotted as lines A and A in FIGS. 6 and 7, respectively.

The above procedure is repeated, except that the polyethylene terephthalate is pinned to the quench drum using an apparatus of the type illustrated in FIG. 4 of the present specification. Oxygen is supplied to the apparatus at a minimum pressure necessary to maintain a continuous stream around the first electrode. The first electrode is a steel wire having a diameter of 0.006 inch. The second electrode is steel and is positioned about 0.6 inch away from the first electrode. The second electrode has an insulating layer of polytetrafluoroethylene resin on the concave surface, except for the terminal end 20 as shown in FIG. 4. The thickness of the insulating layer is about l0 mils. The first electrode is about 0.75 inch away from the surface of the drum. A unidirectional positive current of about 18 kilovolts and about 2 milliamperes is applied to the first electrode. The thickness variations along the transverse and machine direction axes are determined, and the traces plotted and designated B and B on FIGS. 6 and 7, respectively.

A marked improvement in both the transverse and machine direction gauge variation is observed in those films pinned according to the instant process.

EXAMPLE 2 Example '1 is repeated except the polyethylene terephthalate is extruded at a thickness to give a final oriented film having a thickness of 1.42 mil. The transverse direction gauge variations are determined and plotted for the films, and the traces for the film prepared using the Owens et a1. process and that of the instant invention are designated A" and B" respectively in FIG. 8.

EXAMPLE '3 Polyethylene terephthalate is extruded and pinned to a quench drum substantially as described in Example 1 of Owens et al., United States Patent 3,223,757, to establish a control standard. The sheet is extruded at a width of about 40 inches and a thickness of about 7.5 mils, as cast. The filrn is thereafter biaxially oriented to a thickness of about 0.75 mil. The maximum rate of production for films of consistently high quality is determined and designated as R.

The procedure is repeated, using a pinning apparatus of the type illustrated in FIG. 3 of the instant specification. The first electrode is a steel wire having a diameter of 0.006 inch. The second, arcuate electrode is positioned about 0.3 inch away from the first electrode and is insulated therefrom by a 10 mil layer of polytetrafluoroethylene resin. The first electrode is about 0.5 inch away from the drum. A positive unidirectional current of about 2 milliamperes having a voltage of about 18 kilovolts is applied to the first electrode. Oxygen is supplied to the apparatus at a minimal pressure necessary to maintain a continuous stream.

The maximum production rate for the apparatus is determined, and found to be 1.60 R.

EXAMPLES 4-13 In Examples 4 to 13, polyethylene terephthalate is melt extruded at a constant rate onto a cooled quench drum. The quench drum has a diameter of six feet and the extruded sheet is 16% inches wide. The thickness of the 11m on the quench drum varies with the speed of the rum.

below.

Film Maximum Current thickness drum speed Voltage aJin. Example G as (mils) (ft./min.) (kv.) wire) Room air. 7.4 80 9. 2 30 In Examples -7, the procedure of Example 4 is repeated, except that an electrode apparatus of the type illustrated in FIG. 3 is used instead of the bare wire electrode of Example 4. The electrode aparatus is positioned at the same point as the bare electrode, and the same voltage is applied to the wire. In Examples 6 and 7, oxygen and nitrogen are supplied to the apparatus to substantially surround the pinning wire. These gases effect a moderate increase in the amount of ions formed which causes an an intermittent force increase across the width of the film. This causes variations in the touchdown point which actually decreases the maximum operating speed without pinner bubbles.

Film Maximum Current thickness drum speed Voltage a/in.

Example Gas (mils) (IL/min.) (kv.) wire) 5- Room air 6.0 110 9.2 3 6- Oxygen 6.0 105 9. 2 4 7 Nitrogen G. 0 105 9. 2 8

Film Maximum Current thickness drum speed Voltage a/in.

Example Gas (mils) (IL/min.) (kv.) wire) 8 Room air.- 5.0 130 10.5 80 9 Oxygen 5. 0 135 10.8 04 1O Nitrogen 5.0 135 11.3 81

In Examples 11-13, the procedure of Examples 8-10 is repeated, except that the position of the pinning apparatus is adjusted to the best possible position, as opposed to placing the apparatus at the point of best performance of the bare wire.

Film Maximum Current thickness drum speed Voltage a/in Example Gas (mils) (it/min.) (kv.) wire 11 Room air..- 4.75 150 10.1 69 12 Ox'ygen 4. 75 160 10. 9 100 13 Nitrogen 4. 75 160 10. 7 81 I claim:

1. A process for pinning dielectric film to an electrically grounded moving surface which comprises (1) establishing an electrical potential diiference between an electrode pair comprising a first electrode and a second, grounded electrode separate from the moving surface and from the film source, by connecting the first electrode to a high voltage current source, the first electrode being in spaced relationship to the moving surface, the distance between the first and second electrodes being less than the distance between the first electrode and the moving surface and the second electrode being substantially fully insulated from the first electrode by dielectric; and (2) passing the film in proximity to but out of contact with the electrode pair and onto the surface, causing the film to adhere to the surface.

2. A process of claim 1 wherein all of the straight linear paths between the first and second electrodes are intersected by dielectric.

3. A process of claim 1 further comprising substantially surrounding at least the first electrode with a gas in which a Wire current before breakdown of at least about microamperes per inch of wire can be generated, as measured by the current generation test.

4. A process of claim 3 in which the difference between voltage at threshold current and the voltage at spark breakdown of the gas is at least about 2.0 kilovolts, as measured by the current generation test.

5. A process of claim 3 wherein the gas consists essentially of nitrogen.

6. A process of claim 3 wherein the gas consists essentially of oxygen.

7. A process of claim 3 wherein the gas consists essentially of helium.

8. A process of claim 3 wherein the gas comprises a halocarbon selected from at least one of carbon tetrachloride, dichlorodifluoromethane, tetrachloroethane and dichlorotetrafluoroethane.

9. A process of claim 1 wherein the moving surface is a quenching surface, the film source is a melt extruder and the film is melt extruded onto the quenching surface from a die orifice.

10. A process of claim 9 wherein the second electrode is so positioned as to intersect at least about 50% of the straight linear paths between the first electrode and that portion of the web extending from the die orifice to the point of touchdown of the film onto the quenching surface.

11. A process of claim 1 wherein the high voltage current source is a unidirectional current source.

12. A process of claim 11 wherein the current is positive.

References Cited UNITED STATES PATENTS 3,427,686 2/1969 Busby 1815 M 3,196,063 7/1965 Paquin et al. 26422 3,520,959 7/1970 Busby 264-22 3,470,274 9/1969 Sandiford et al. 264-22 ROBERT F. WHITE, Primary Examiner G. AUVILLE, Assistant Examiner U.S. Cl. X.R. 264-85, 216 

